Automotive engineers have placed continued emphasis on the reduction of vehicle noise, vibration and harshness (NVH), to ensure a smooth vehicle operation. Tire performance is a key factor in controlling vehicle NVH. To that end, it is desirable to obtain tire contact force distribution data so that the data may be used in computer modeling to improve overall vehicle NVH.
A typical apparatus for measuring tire contact force distribution, as disclosed in Chananel Kvatinsky et al., "Application of the Contact Pressure Display (CPD) Method to Tire Mechanics", SAE Technical Paper 851627, Sep. 1985, employs a multiplicity of pressure sensors embedded in a tire contact member such that when the tire is pressed onto the pressure sensors, the contact force distribution may be determined. Discrete pressure sensors, being located at discrete positions in the contact member, do not provide a full-field display to determine the contact force distribution throughout the entire footprint. It is to be appreciated that "footprint" means that portion of the tire in contact with the contact member.
FIG. 1 shows the results of the contact force distribution of a tire using the prior art contact pressure display apparatus described above. As shown in FIG. 1, a grid of circles, 2, represents the pressure distribution of the tire. The circles represent positions where the tire surface contacts the apparatus. Thus, voids 4 between circles 2 represent areas where no data is obtained. As a result, this apparatus lacks full-field capability. In addition, this prior art apparatus might result in error caused by stress concentration because the apparatus measures only normal forces at discrete points.
Another apparatus for measuring displacement, as disclosed in Piotr J. Wesolowski et al., "Pulsed Holographic Interferometry in Predictive Tire Testing", 2545 Int'l Soc'y Optical Engineering Proc. 33 (1995), uses pulsed holographic interferometry. FIG. 2 represents the results using this prior art apparatus. Contour lines 6 represent lines of constant tire displacement normal to the footprint plane. Because the tire is pressed against a rigid surface, the displacement of the tire in the area defined by the contact patch 8 is constant. That is why FIG. 2 shows a single contour plane at the tire contact patch 8. To measure full-field contact force, i.e. the distribution force of the tire in the contact patch, there must be a variation in tire displacement in the contact patch. According to this prior art apparatus, there is no variation in displacement in the contact patch due to the tire being pressed against a rigid surface, and therefore this prior art apparatus lacks full-field capability.
A third prior art apparatus for measuring contact force distribution is disclosed in U.S. Pat. No. 5,092,166. The apparatus of the '166 patent uses a television camera to measure the deformation of elastic projections resulting from pressing of the tire onto a light-colored elastic sheet. The elastic projections are semispherical in shape and extend from the sheet. The sheet is disposed between a glass plate and the tire and, as the tire presses into the sheet, the projections deform. The camera records the amount of deformation to determine the contact force. However, since the projections are semispherical in shape, there must be voids between the projections. As a result, no contact force information can be obtained at these locations. Thus, this prior art apparatus also lacks full-field capability.